Alternating current (AC) motors are widely used in industry to drive rotating machinery. The vast majority of these applications require the motor to run at a constant angular velocity corresponding to the frequency of the input AC power. At present, the number of induction motors in commercial use exceeds the number of synchronous motors used for the same type of duty. There are two primary reasons for the preference of induction motors over synchronous motors. Compared to synchronous motors, induction motors exhibit ruggedness and simplicity in their design. Induction motors also have the ability to be started by direct connection to the AC power line.
Notwithstanding the above mentioned benefits of the induction motor over the synchronous motor, there are certain favorable attributes of synchronous motors which suggest that they should be used instead of induction motors. For example, synchronous motors have greater efficiency than induction motors. This is particularly true of permanent magnet synchronous motors which have no electrical power losses in the rotor. Suitable permanent magnet rotors are currently being developed for this purpose. Although the cost of such a type of synchronous motor would exceed the cost of a conventional induction motor, it should be possible to recover the cost differential over a period of time in the form of energy savings.
However, despite the advantageous attributes of the synchronous motor, a primary limitation and disadvantage of the synchronous motor and, in particular, permanent magnet synchronous motors, is the inherent difficulty of starting the synchronous motor. For example, the direct on-line starting process routinely used for induction motors produces extremely large oscillating torques in the permanent magnet synchronous motor. These oscillating torques do occur in induction motors, but are not so large as to cause mechanical failure. However, it has been found that these oscillating torques may be large enough to cause structural damage to the permanent magnet synchronous motors or the machinery driven thereby. Yet another limitation of the direct on-line starting of a permanent magnet synchronous motor is the undesirable, large, asynchronous current components induced in the AC power lines during start up.
To overcome these disadvantages and limitations of synchronous motors, some prior art synchronous motors have been provided with windings on the rotor wherein the windings provide induction motor reaction for starting directly on-line. These rotor windings also provide a damping action when the motor is running synchronously. A significant disadvantage and limitation of this type of synchronous motor is that the rotor windings significantly add to the complexity and cost of the motor, thereby negating benefits gained by increased efficiency.